The present invention relates to an anti-frost system for a windshield of a motor vehicle, and, more particularly, to an anti-frost system of a windshield wherein general-purpose sensors can be used as sensors for detecting frosting conditions.
Various anti-frost systems for the windshield of motor vehicles have been proposed. For instance, in an anti-frost system actually in use, frost accumulated over the surface of the windshield is melted by electric power supplied from an alternator power supply mounted on the vehicle. In this system a thin metal film resistor is laminated with windshield sheets and the resistor is energized by turning on a switch after the vehicle engine has been started.
The above mentioned system is a defrosting system. Because the windshield must be heated above 0.degree. C., an electric power supply system (an alternator and regulator) capable of supplying high voltage and high power (for instance, 50-60 V and more than 1500 W) must be mounted on the vehicle so that changes to the specifications of the power supply system must be made. An example of such a system is disclosed in Japanese Patent Laid-Open (Unexamined) Publication No. 58-174,046.
In view of the above, there has been proposed an anti-frost system for preventing accumulation of frost in advance in Japanese Patent Laid-Open (Unexamined) Publication No. 1-114,559. This system is not a defrosting system. According to the system, when the temperature and humidity in the atmosphere surrounding the windshield become a condition involving a possibility of frosting, a heating element incorporated in the windshield is energized through a controller, thereby preventing the accumulation of frost on the windshield in advance.
The known system stated above has a temperature sensor and a humidity sensor. These sensors are special sensors. For example, the humidity sensor is a special sensor for sensing humidity on the windshield and must be installed on the windshield. Output signals from these sensors are delivered to the controller. When the temperature and humidity conditions predict a possibility of frosting on the windshield, the controller operates to energize the heating element incorporated in the windshield.
The known system needs special sensors as mentioned above. In order for these sensors to correctly detect the weather conditions, the places at which these sensors are installed are strictly limited. Because of the construction of motor vehicles, ideal places cannot be freely selected so that sometimes conditions of frosting are not properly detected. In other words, the limitation to the places at which the sensors are installed presents a bottleneck for correct detection of weather conditions.
Moreover, in addition to the battery for the engine, another battery must be mounted on the vehicle solely for the purpose of energizing the heating element in the windshield, or a large-sized battery must be used for the same purpose.